FIG. 1 shows a cross sectional view of a basic proportional counter. Cathode 10, which is shown, for simplicity, as cylindrical, surrounds thin wire anode 12 and forms a chamber which contains a suitable gas. Cathode 10 is made thin enough or window means may be provided so that radiation of interest may enter the chamber. In the chamber the radiation will react with the gas to form ion pairs. Electrons released by this pair formation are accelerated towards the positively charged anode, while the positive ion formed is accelerated more slowly towards the cathode. As the electrons approach the anode, they acquire sufficient energy to form other ion pairs. The electrons released by the secondary pair formations in turn form still more pairs and so on. This phenomena is commonly referred to as "avalanche formation." When these avalanche electrons reach the anode, and as the positive ions formed move towards the cathode, they cause an electrical signal which may be detected by an electronic means, not shown. By a proper choice of the potential of the anode with respect to the cathode, the magnitude of the signal will be proportional to the energy of the incident radiation, hence the name "proportional counter."
When measurement of the energy of the incident radiation is not a factor such devices may be operated with a higher potential or a different gas so that the signal is not directly proportional to the energy. Such a mode of operation is known to those skilled in the art of nuclear instrumentation as operation in the limited proportional region. Hereinafter, it is to be understood that all devices and methods described may be operated in the "limited" as well as the directly proportional region.
The principle of the proportional counter may be used in a radiation detector designed to localize in space the occurrence of radiation. FIG. 2 shows a standard multiwire proportional chamber, one device which may be used for this purpose. The spaced parallel planar cathodes 20 and 22 now form the boundaries of the chamber. Avalanches caused by radiation incident upon the chamber will be detected by the closest of anodes 24. Thus, if the spacing between cathodes 20, 22 is S and the distance between anodes 24 is D, a signal occurring on a particular anode will serve to localize the occurrence or radiation approximately within a D by S rectangle centered on the particular anode. Means for localizing the event in the orthogonal direction are well known to those skilled in the art of nuclear instrumentation, and need not be further discussed here.
Since the spacing in a multiwire proportional chamber is typically on the order of two millimeters, leading to a large number of anode wires and a high cost for the associated electronics in a typical device, it is desirable to provide a means for localizing radiation events having a greater spacing between anode wires. FIG. 3 shows a typical drift chamber, one means having such an increased spacing. Spaced planar cathodes 30 and 32 serve to bound a gas-filled chamber containing anode wires 38. Radiation which forms ion pairs within the chamber continues on and is detected by detection means 40. Scintillation counters are a suitable detection means where the ionizing radiation consists of charged particles. Other detection means suitable for electromagnetic and neutral radiation are well known to those skilled in the art of nuclear instrumentation and need not be discussed further here. Since ionizing radiation travels at high speeds compared to the speeds at which electrons move to the anode, the output signal of the detection means 40 approximately establishes the time at which the ion pair was formed. Thus the difference between the times of the output signal of detecting means 40 and the detection of a signal on a particular anode wire 38 is a measure of the approximate distance between the point where an ion pair is formed and the anode wire. The accuracy of the drift chamber is improved by addition of potential wires 36 which serve to insure a more uniform field around anode wires 38.
Details of a design, construction and use of multiwire proportional chambers and of drift chambers are well known to those skilled in the art of nuclear instrumentation and do not require further discussion here. However, it may be seen by examining the symmetry of the structures shown in FIGS. 2 and 3 that there is an inherent ambiguity as to whether an ionizing event occurs to the left or to the right of a particular anode wire. One method for resolving this ambiguity, known as the "double wire" method, is to replace each of anode wires 38 with a closely spaced pair of wires, each having its own associated electronics. Another method for resolving this ambiguity, known as the "double chamber" method, is illustrated in FIG. 4. Double chamber 48 comprises adjacent drift chambers 44 and 46, so arranged that incident radiation causes ionization events in each of chambers 44 and 46. Because of the arrangement of chambers 44 and 46, these ionization events are detected by unique pairs of anode wires 50.
It will be obvious that each of these methods of resolving the left/right ambiguity involves an increased complexity of the chamber structures as well as requiring an increased amount of electronics. Further problems arise for "double chamber" type detectors for radiation tracks not normal to the chamber plane.
The subject invention substantially overcomes the above-described left/right ambiguity problem by means of an improved multiwire chamber wherein the improvement comprises; a pair of localizing wires coextensive with and spaced from each of the anode wires in said multiwire chamber, one of said localizing wires being located on either side of said anode wire; and, differential amplifier means for detecting the signal difference on each wire of said pair. (By multiwire chamber herein is meant a chamber as described hereinabove, having a plurality of anode wires, whether such a chamber be of the multiwire proportional chamber type or of the drift chamber type.) In the subject invention, the left/right information is obtained from the polarity of the difference signal produced by the differential amplifier. In practice, a short signal is desired in order to process high counting rates, while a signal as large as is practicable is desired to permit a clear left/right distinction. To balance these objects, a difference signal is differentiated with a time constant which is matched to the particular application. In general, the design of the differential amplifier and the associated electronics will be obvious for persons skilled in the art of nuclear instrumentation.
The above-described localizing wires may also serve other functions in the multiwire chamber. In one embodiment of the subject invention the multiwire chamber may be a drift chamber and the localizing wires may be potential wires.
In a second embodiment invention the multiwire chamber may be a multiwire proportional chamber, and the localizing wires for each anode wire may be the adjacent anode wires.
In another embodiment of the subject invention, the multiwire chamber may be a drift chamber, having a large spacing between anode wires. In such a drift chamber, field shaping electrodes are provided in close proximity to the cathodes. These field shaping electrodes may serve as localizing wires in this embodiment.
Therefore, it is an object of this invention to provide an improved multiwire chamber for the detection and localization of radiation, said chamber having means for resolving the left/right ambiguity, whereby the error in localization is reduced by a factor of 2.
It is another object of the subject invention to provide such an improved multiwire chamber which has a minimal increase in the complexity of the chamber structure and a minimal increase in the amount of required electronics.
It is another object of the subject invention to provide a multiwire chamber, having the capability to resolve the left right ambiguity and which has reduced sensitivity to the angle of incidence of the radiation being detected.
It is still another object of the subject invention to provide an improved multiwire chamber, having the capability to resolve the left/right ambiguity, which is low in cost. Other objects of the subject invention will become apparent in the discussion hereinafter.